All electro-acoustic transducers provide some degree of undesired change in acoustical waveform, i.e. distortion. Distortion is commonly divided into three types, i.e. frequency distortion, amplitude distortion (harmonic distortion) and phase distortion. Harmonic and phase distortion are particularly troublesome when the output acoustical transducer is located in a confined space or cavity, for example, in a hearing aid, sound head set or telephone receiver. Often the cavity has a "tuned" frequency and the materials in the cavity have resonant frequencies which, when coupled with the waveform from the transducer, result in peaks or spikes in the waveform corresponding to harmonic frequencies of the waveform, i.e. harmonic distortion. Additionally, phase shifts in the waveform can occur which produce distortion and also result in harmonic distortion where the resonant peaks persist for a period of time after the desired pulse. These effects can be heard by the listener as an annoying ringing.
With amplified hearing devices, such as hearing aids, the input signal is amplified so that the wearer of the aid receives an amplified signal which should correspond to the waveform of the input signal. However, noise and other extraneous signals are also amplified which create a problem of clarity and make it difficult for the wearer of the aid to "focus" on the desired sound. It has recently been found that much of the difficulty associated with focusing is due to the presence of harmonic distortion in the amplified signal. Minimizing harmonic and phase distortions provide much greater clarity in the amplified sound and allow the listener using the amplification device to more readily focus on the desired sounds.
Feedback to the input transducer can also be a problem with devices such as hearing aids in which the input transducer is located in close proximity to the output transducer. The amplification of such feedback or resonance waves can result in "ringing" which can be unpleasant to the wearer of the hearing aid. Additionally, in the real world, the amount of usable gain available with a hearing aid varies depending upon the complexity of the signal being amplified. H. C. Schweitzer, Hearing Instruments, Volume 37, Nos. 1 and 2, 1986. Consequently, distortion can result in a lower output of the hearing aid and, therefore, a lower usable gain available for amplification.
To date, efforts to remedy the problems associated with lack of clarity and distortion in amplified signals have primarily focused on modifications in the electronic components and in physical placement of such components. The effect that materials of construction might have were considered in the past but were not found to be significant. It has been reported that: "Ear mold material was once considered a factor in the acoustic performance [of] ear molds. Except for the way in which the material might influence the tightness of the seal in the ear canal, it appears insignificant acoustically." S. F. Lybarger, "Earmolds", Handbook of Clinical Audiology, J. Katz, Editor, 1972, The Williams and Wilkin Company. Materials such as sintered pellets, mesh screens, lamb's wool and cotton have been used as acoustic obstructions (filters) in the earmold and earphone tubing or earhook to increase acoustic resistance to modify response peaks. Hearing Aid Assessment and Use in Audiologic Habilitation, William R. Hodgson, Editor, 3rd Ed. p. 85 and p. 93 (1986). These obstructions, however, can cut down gain and output and can add distortion at higher acoustic pressures.
Although great advances have been made in providing electronic components which reduce distortion and improve clarity, lack of clarity in the output of acoustical transducers and difficuties associated with focusing on sounds continue to be problems. Consequently, there is a need for methods and apparatus for minimizing the distortions which can occur when acoustic waveforms are generated particularly in a confined space.
It has now been found that the above-described problems associated with harmonic distortion and feedback can be minimized by the use of a plurality of microspheres located in relation to the output transducer to interact with interfering waveforms. When the microspheres are used, a significant decrease in the total harmonic distortion is achieved particularly when the transducer or its output waveforms are in a confined space, such as an ear canal. Additionally, when the microspheres are used in the shell or housing to contain the input and output transducers, amplifier and associated electronics, significant reductions in total harmonic distortion and feedback are observed.
Both solid and hollow microspheres are well known as fillers in the plastics industry. They are commonly used as extenders and the hollow microspheres find application where it is desirable to reduce the weight of the polymeric product and improve stiffness and buoyancy. However, the reduction in distortion of acoustic waveforms is totally unexpected in view of the reported acoustical properties of cellular polymers. As set forth in the Encyclopedia of Polymer Technology:
"The acoustical properties of polymers are altered considerably by their fabrication into a cellular structure. Sound transmission is altered to only a minor extent since it depends predominantly upon the density of the barrier (in this case, the polymer phase). Cellular polymers are, therefore, very poor materials to use by themselves in order to produce sound transmission. They are quite effective in absorbing sound waves of certain frequencies (24). Materials with open cells on the surface are particularly effective in this respect."
Encyclopedia of Polymer Science and Technology, Herman F. Mark, Editor, 1970. As set forth in Volume 12, page 716, of the same series, "open-celled foams provide good sound deadening whereas hard, closed-cell foams have only slight absorption"; and on page 706, "since widespread friction of the air in the foam is important, closed-celled foamed polymers are in general not suitable for air-borne sound absorption." Accordingly, there is no suggestion in the known art of the advantages of the instant invention.